International Patent Publication No. WO2010/046307 and International Patent Publication No. WO2010/110343 disclose conventional technique for a dual clutch transmission (DCT) that includes two gear shafts, for odd-numbered speed stages and even-numbered speed stages, and a speed-change gear mechanism relating thereto, in a hybrid vehicle that includes an internal combustion engine and an electric motor as a drive source. According to these publications, a first speed-change gear mechanism provided on a first input shaft (namely, a first gear shaft) that is connected/disconnected to the internal combustion engine via a first clutch is provided with drive gears for odd-numbered speed stages (for example, third- and fifth-speed stages) that is selectively connectable to the first input shaft (gear shaft) via a synchronizer (synchromesh mechanism), and driven gears that mesh with the respective drive gears are provided on a countershaft (speed-change output shaft). Further, a second speed-change gear mechanism provided on a second input shaft (namely, a second gear shaft) that is connected/disconnected to the internal combustion engine via a second clutch is provided with drive gears for even-numbered speed stages (for example, second- and fourth-speed stages) that is selectively connectable to the second input shaft (gear shaft) via a synchronizer (synchromesh mechanism), and driven gears that mesh with the respective drive gears are provided on the countershaft (speed-change output shaft). Meanwhile, one end of the first input shaft (gear shaft) is connected to the electric motor and the planetary gear mechanism is also disposed on the one end of the first input shaft (gear shaft) to function as a first-speed gear stage.
FIG. 7 is a skeleton diagram that illustrates some parts relating to the planetary gear mechanism that have been extracted from the description of the conventional dual clutch transmission above. In the figure, reference numeral 1 denotes the first input shaft (gear shaft), 2 denotes the second input shaft (gear shaft), and 3 denotes the countershaft (speed-change output shaft). A third-speed drive gear 13 and a fifth-speed drive gear 15 are concentrically provided on the first input shaft 1 so as to be relatively rotatable thereto. A second-speed drive gear 22 and a fourth-speed drive gear 24 are concentrically provided on the second input shaft 2 so as to be relatively rotatable thereto. On the countershaft 3, a driven gear 31 that meshes with the third-speed drive gear 13 and a driven gear 32 that meshes with the fifth-speed drive gear 15 are fixed to rotate integrally therewith. In addition, the driven gear 31 meshes with the second-speed drive gear 22 and the driven gear 32 meshes with the fourth-speed drive gear 24. A synchronizer (namely, a synchromesh mechanism) 51 is provided to selectively transmit rotation of the first input shaft 1 to either of the third-speed drive gear 13 and the fifth-speed drive gear 15. A synchronizer (synchromesh mechanism) 52 is provided to selectively transmit rotation of the second input shaft 2 to either of the second-speed drive gear 22 and the fourth-speed drive gear 24.
A planetary gear mechanism 40 is provided on one end of the first input shaft (gear shaft) 1. A sun gear 41 of the planetary gear mechanism 40 is coupled to the first input shaft (gear shaft) 1 and the sun gear 41 is also coupled to a rotor of an electric motor MOT. In the planetary gear mechanism 40, the planetary gears 42 are engaged with the sun gear 41. A carrier 43 of the planetary gears 42 is fixed to the third-speed drive gear 13 such that the carrier 43 constantly rotates integrally with the third-speed drive gear 13, and the third-speed drive gear 13 is engaged with the third-speed driven gear 31 of the countershaft 3. As a result, the carrier 43 of the planetary gears 42 is dragged by rotation of the countershaft 3 even when a drive power is not acting thereon, such that the carrier 43 is constantly rotating along with the countershaft 3. In the planetary gear mechanism 40, a ring gear 44 is engaged with the planetary gears 42. The ring gear 44 is arranged such that it can be selectively fixed via the synchronizer (synchromesh mechanism).
When selecting a first-speed stage, the ring gear 44 is fixed to a case (not shown) of the transmission via the synchronizer (synchromesh mechanism) 50 and the synchronizer (synchromesh mechanism) 51 is maintained in a neutral state. Accordingly, the rotation of the first input shaft 1 is transmitted to the countershaft 3 via the sun gear 41, the planetary gears 42, the carrier 43, the third-speed drive gear 13 and the third-speed driven gear 31. When any of speed stages other than the first-speed stage is selected, the synchronizer (synchromesh mechanism) 50 is disengaged and the ring gear 44 constantly rotates freely in accordance with the rotation of the countershaft 3.
In the aforementioned prior art, a speed of the ring gear 44 is increased in accordance with differential rotation between the sun gear 41 to which the rotation of the first input shaft 1 is constantly transmitted and the carrier 43 to which the rotation of the countershaft 3 is constantly transmitted. In particular, in a high vehicle speed region, despite the fact that the planetary gear mechanism 40 is not involved in changing speeds, a large differential speed is generated due to rotation caused by dragging, which results in unnecessary rotation loss as well as generation of heat due to rotational friction.